The present invention generally pertains to signal communications and is particularly directed to communications between transponders.
A typical prior art transponder includes means for receiving an interrogation signal and means for transmitting a response signal in response to receipt of the interrogation signal. In some transponder systems, a group of transponders are interrogated by an interrogation signal transmitted by a relatively high-power transmitter under the control of a single controller and the resultant response signals are received and monitored at the same location as the high-power transmitter.
Transponders are used for retrieving data pertaining to objects to which the transponders are attached. In some transponder systems, the retrieved data is provided to the transponders by attached sensors and then included in the response signal transmitted in response to receipt of an interrogation signal.
In other transponder systems, such as those used to detect theft attempts in retail stores, the transponder is attached to merchandise and transmits a response signal indicating the presence of the transponder and the merchandise attached thereto whenever the transponder is within an interrogation zone in which an interrogation signal can be received, which zone typically is located at a store exit.
In still other transponder systems, the transponder is programmed to respond to an interrogation signal by transmitting a response signal that identifies the object to which the transponder is attached. One such transponder system is used to identify and track the location of airborne aircraft. Another such transponder system utilizes transponders that are known as RFID tags. RFID tags are self contained and battery operated and include a microcontroller for data processing management.
RFID tags are used to identify objects to which the RFID tags are attached when such objects are within an interrogation zone in which the interrogation signal can be received. RFID tags are also used for safety and security applications. RFID tags are attached to many different types of objects. It is particularly advantageous to attach the RFID tags to cargo containers in order to track the location of the cargo containers that can be identified by the RFID tag transponder response signal as the cargo containers attached thereto are moved from one interrogation zone to another.
Problems in communicating with all of transponders that are respectively attached to a group of cargo containers sometimes arise when the containers of the group are stacked and/or closely situated on decks of ships or in freight yards. This problem arises because the usual stacking and close positioning of the containers sometimes either prevents some of transponders attached to the closely situated containers from receiving the interrogation signals, or prevents all of the response signals transmitted by the transponders attached to the containers from emerging out of the closely situated group of containers so that each response signal can be received and monitored.